The Zintl-Klemm concept and its broader extensions

Abstract

The Zintl-Klemm concept (ZKC) remains among one of the most important and useful concepts developed in solid state chemistry. Based on the deeply rooted chemical knowledge of electron counting and electronegativity difference, it has allowed to understand crystal structures involving main group elements (aluminates, silicates, phosphates, etc.) as well as structures of alloys (Ni2In, Co2Si, LiAlSe2, CsLiSO4, etc.) which had not been previously explained. Along this chapter we will discuss the principles of the ZKC making emphasis on its extension to complex structures of oxides. Surprisingly, the ZKC holds in the cation arrays of oxides despite of being embedded in an oxygen matrix as it is the case of aluminates, silicates, and germanates among others. Up to now, these compounds had been only classified taxonomically, their structures remaining unexplained. The extended ZKC (EZKC) considers that electrons are transferred from the donor cations to the atoms forming tetrahedral oxopolyanion skeletons. The cation subarrays of these compounds behave as true Zintl phases and the tetrahedral skeletons are true Zintl polyanions. Selected examples are provided to illustrate how Zintl polyanions found in these complex structures resemble the geometry of their parent block-p elements or simple molecules formed with these elements. Specific mention is made to the amphoteric character of atoms like Na, Ni, Si, Al and Ge in order to achieve stable Zintl polyanion configurations. To conclude the chapter, we summarize how the application of the EZKC reveals the presence of preferred skeletons as building blocks of complex structures, making possible to establish a link between chemical composition, structure and atomic octet fulfillment.